In the recent years there has been a large growth in the usage of wireless devices and systems that operate in unlicensed ISM-like spectra (ISM=Industrial, Scientific and Medical) as e.g. defined by the ITU Radiocommunication Sector (ITU-R). Examples are Wi-Fi-, Bluetooth-devices, home automation devices, etc. The volume of the unlicensed wireless devices and the variety of their applications is expected to grow further in the coming years, including in the field of medical devices, e.g. hearing aids, etc. However, the largest portion of the unlicensed devices in the future is likely to belong to the Radio Frequency Identification (RFID) systems. It is expected that the RFID readers and networks will be deployed everywhere in order to communicate with the billions of RFID tags attached to humans, objects, animals, etc.
The basic paradigm for operation in the unlicensed bands is that the devices should apply techniques in order to avoid radio interference with signals (a) received from other devices and (b) transmitted towards other devices. For example one mechanism to achieve that can be the following: If a device that wants to transmit detects that there is an ongoing transmission, it backs off and does not start its transmission for some (random) time. This is the principle of “Listen-Before-Talk”. In another example, a device that detects that there is an ongoing transmission on Channel M, will switch to a different Channel N and try to perform the communication in that channel. In general, the devices are trying to access the spectrum efficiently, which means that we want to achieve a situation where as many devices as possible are succeeding to simultaneously send their information successfully to the intended receivers.
An example of an efficient spectrum usage is outlined in the following. There are two persons, A and B, and each of them wants to have his or her headset to communicate with his or her mobile phone. The communication is done by sending data packets from the headset to the mobile and vice versa. If these two persons are standing next to each other, then it is not possible to transmit packets for the person A and person B simultaneously (on the same communications channel), because they will be destroyed in mutual interference. One way to go around this is that the mobile of A agrees with the mobile of B to use the communication channel by time sharing and agree in which time periods the devices of A will use it and in which period the devices of B will use it. Another way would be that A and B agree to use different channels and thus avoid the mutual interference.
However, achieving such a spectrally efficient operation is a difficult problem. In the previous example, it is likely that the mobiles of A and B are not able or not willing to communicate with each other, and thus they cannot agree on how to share the usage in time or to shift channels. Therefore, there is a lot of ongoing research into the techniques that facilitate efficient spectrum sharing and utilization. This requirement becomes particularly important for the 863-870 MHz band, as the number of short-range devices (SRDs) in that region will proliferate in the coming years, including future medical devices, such as hearing instruments.
A particular problem in the 863-870 MHz band is that the short-range devices will have to share the wireless medium with the transmission of the RFID reader (interrogator) devices. This is because the RFID interrogators will use much larger power than the short range devices. This is causing an undesirable asymmetric situation: Many SRDs will suppress their transmission or will be compelled to change the communication channel after detecting the interference caused by the RFID readers, while, on the other hand, their transmission will likely not disturb the communication within the RFID system.
US 2007/0063818 deals with a radio-frequency identification system comprising a radiofrequency identification tag and an interrogator. In one embodiment, the interrogator is configured to determine a nature of a received signal in a frequency channel and to selectively enable transmission of an interrogation signal in the frequency channel based on the determined nature of the received signal. In another embodiment, the interrogator is configured to select an interrogation frequency channel based on whether interference is likely to occur due to signals in the selected channel and/or signals in adjacent frequency channels.